Smart Contract Development: A Complete Technical Guide for 2026

Smart contracts are programs that run on a blockchain. They execute automatically when conditions are met, cannot be stopped or censored, and (for most implementations) cannot be modified after deployment. These properties make them powerful for trustless applications — and high-stakes from a security perspective.

This guide covers practical smart contract development: patterns, tools, testing, security, and deployment.

Core Solidity Patterns

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

/**
 * @title Escrow
 * @notice Holds funds until conditions are met, then releases to recipient
 * @dev Demonstrates: access control, reentrancy guard, safe token transfers, events
 */
contract Escrow is Ownable, ReentrancyGuard {
    using SafeERC20 for IERC20;

    enum State { AWAITING_PAYMENT, AWAITING_DELIVERY, COMPLETE, DISPUTED }

    struct Deal {
        address buyer;
        address seller;
        address token;        // address(0) for native ETH
        uint256 amount;
        State state;
        uint256 deadline;
        bool buyerApproved;
        bool sellerApproved;
    }

    mapping(uint256 => Deal) public deals;
    uint256 public dealCount;

    event DealCreated(uint256 indexed dealId, address buyer, address seller, uint256 amount);
    event PaymentDeposited(uint256 indexed dealId);
    event FundsReleased(uint256 indexed dealId, address recipient, uint256 amount);
    event DisputeRaised(uint256 indexed dealId);

    modifier inState(uint256 dealId, State expectedState) {
        require(deals[dealId].state == expectedState, "Invalid state");
        _;
    }

    function createDeal(
        address seller,
        address token,
        uint256 amount,
        uint256 deadlineDays
    ) external returns (uint256 dealId) {
        dealId = ++dealCount;
        deals[dealId] = Deal({
            buyer: msg.sender,
            seller: seller,
            token: token,
            amount: amount,
            state: State.AWAITING_PAYMENT,
            deadline: block.timestamp + (deadlineDays * 1 days),
            buyerApproved: false,
            sellerApproved: false
        });
        emit DealCreated(dealId, msg.sender, seller, amount);
    }

    function deposit(uint256 dealId)
        external
        payable
        nonReentrant
        inState(dealId, State.AWAITING_PAYMENT)
    {
        Deal storage deal = deals[dealId];
        require(msg.sender == deal.buyer, "Not buyer");

        if (deal.token == address(0)) {
            require(msg.value == deal.amount, "Incorrect ETH amount");
        } else {
            IERC20(deal.token).safeTransferFrom(msg.sender, address(this), deal.amount);
        }

        deal.state = State.AWAITING_DELIVERY;
        emit PaymentDeposited(dealId);
    }

    function approve(uint256 dealId) external inState(dealId, State.AWAITING_DELIVERY) {
        Deal storage deal = deals[dealId];
        if (msg.sender == deal.buyer) deal.buyerApproved = true;
        if (msg.sender == deal.seller) deal.sellerApproved = true;

        if (deal.buyerApproved) {
            _release(dealId, deal.seller);
        }
    }

    function _release(uint256 dealId, address recipient) private nonReentrant {
        Deal storage deal = deals[dealId];
        deal.state = State.COMPLETE;
        uint256 amount = deal.amount;
        deal.amount = 0; // CEI pattern — update state before transfer

        if (deal.token == address(0)) {
            (bool success,) = recipient.call{value: amount}("");
            require(success, "ETH transfer failed");
        } else {
            IERC20(deal.token).safeTransfer(recipient, amount);
        }

        emit FundsReleased(dealId, recipient, amount);
    }
}

Critical Security Patterns

Checks-Effects-Interactions (CEI) — always validate inputs first, update state second, call external contracts last. The reentrancy vulnerability in the infamous DAO hack came from calling an external contract before updating the internal balance.

ReentrancyGuard — use OpenZeppelin's ReentrancyGuard on all functions that transfer value.

SafeERC20 — not all ERC-20 tokens return boolean on transfer (USDT does not). SafeERC20 handles this correctly.

Integer overflow — Solidity 0.8.0+ has built-in overflow protection. For older code, use SafeMath.

Testing with Hardhat

// test/Escrow.test.ts
import { expect } from "chai"
import { ethers } from "hardhat"
import { loadFixture } from "@nomicfoundation/hardhat-toolbox/network-helpers"

describe("Escrow", () => {
  async function deployFixture() {
    const [owner, buyer, seller] = await ethers.getSigners()
    const Escrow = await ethers.getContractFactory("Escrow")
    const escrow = await Escrow.deploy(owner.address)
    return { escrow, owner, buyer, seller }
  }

  it("should release funds when buyer approves", async () => {
    const { escrow, buyer, seller } = await loadFixture(deployFixture)
    const amount = ethers.parseEther("1.0")

    // Create and fund deal
    await escrow.connect(buyer).createDeal(seller.address, ethers.ZeroAddress, amount, 7)
    await escrow.connect(buyer).deposit(1, { value: amount })

    const sellerBalanceBefore = await ethers.provider.getBalance(seller.address)
    await escrow.connect(buyer).approve(1)
    const sellerBalanceAfter = await ethers.provider.getBalance(seller.address)

    expect(sellerBalanceAfter - sellerBalanceBefore).to.be.closeTo(
      amount, ethers.parseEther("0.001") // Account for gas
    )
  })

  it("should prevent reentrancy attacks", async () => {
    // Deploy malicious contract and attempt reentrancy
    // ...
  })
})

Deployment and Verification

# Deploy to testnet
npx hardhat run scripts/deploy.ts --network sepolia

# Verify on Etherscan
npx hardhat verify --network sepolia DEPLOYED_ADDRESS "constructor_arg1"

Always deploy to a testnet first. Always verify source code on Etherscan — it builds trust and enables others to audit your code.

Need smart contracts built and audited? Viprasol builds and audits Solidity smart contracts. Contact us.

See also: Blockchain Development Company Guide · DeFi Development Guide (coming soon)